Ring expansion reactions of anti-aromatic boroles: a promising synthetic avenue to unsaturated boracycles

Abstract

Conjugated boron heterocycles have emerged as attractive synthetic targets due to their potential in medicinal chemistry and as electronic materials. However, the development of unsaturated boracycles has been hampered by difficulties in their preparation. Recently, a new synthetic avenue to access these species has been developed that takes advantage of the high reactivity of boroles. These five-membered anti-aromatic heterocycles can react with substrates to furnish ring expansion products via the insertion of one, two, or three atoms into the boracyclic ring. The ring expansion can occur via two pathways, the first exploits the activated diene moiety of the heterocycle in Diels-Alder chemistry with the resulting bicyclic species undergoing further rearrangements. The second reaction pathway is initiated by the coordination of the Lewis basic site of a substrate to the highly Lewis acidic boron center rendering the endocyclic B-C bond of the borole nucleophilic, inducing the formation of larger boracycles via attack at the electrophilic site of the substrate. This review summarizes the current state of this chemistry and details the mechanisms leading to the products. The methodologies described herein could very well be extended to other substrates, as well as applied to other anti-aromatic heterocycles.

Recent Developments in Azaborinine Chemistry

“…With the less sterically crowded trimethylsilyldiazomethane, the diazo carbon atom insertion occurs, yielding a boracyclohexadiene ( 14 ) (Scheme ) . These cycloaddition reactions were reviewed within the context of other borole ring expansion reactions …”

Preparation and Characterization of a π‐Conjugated Donor–Acceptor System Containing the Strongly Electron‐Accepting Tetraphenylborolyl Unit

“…In order to examine the influence of the strongly accepting borole unit on the electronic and photophysical properties of 1 (vide infra), it was further derivatized to form an aromatic 1,2‐azaborinine group (Scheme ) . This transformation can be completed in one step by using an organic azide . Treatment of 1 with an excess of trimethylsilyl azide turned the initial yellow solution colorless.…”

Electronic Push–Pull Modulation by Peripheral Substituents in Pentaaryl Boroles

“…Being an isolable antiaromatic compound class, boroles have received growing interest in recent years . Due to their peculiar electronic situation, they feature intriguing reactivity including E−H bond splitting, cycloadditions and ring expansions, as well as Lewis acidity enabling adduct formation even with weak Lewis bases such as CO …”